Significant improvements in fiber materials and spinning processes allowed spinning of multifilament yarns with high initial modulus and low shrinkage, and numerous processes and compositions have been developed in recent decades.
For example, Davis et al. describe in U.S. Pat. No. 4,101,525 a spinning process in which a dimensionally stable industrial strength multifilament polyester yarn is formed by extruding PET-containing polyester filaments using a solidification zone almost immediately after the spinneret under high spin-line stress to achieve almost immediate solidification of the filaments to produce yarns with relatively high birefringence. The so formed undrawn yarn is then drawn and further processed. Such processes generally allow formation of dimensionally stable yarns, however, present various difficulties in their formation. Most significantly, as the crystallinity increases, drawability of the yarns often significantly deteriorates.
In another example, Saito et al. describe in U.S. Pat. No. 4,491,657 formation of a dimensionally stable polyester yarn, in which the filaments are subjected to a heating zone operated at about the melting point of the polymer followed by a rapid cooling zone. The so formed filaments are then drawn and twisted into cords with relatively desirable properties. However, in most practical applications Saito's process is generally limited to yarns with relatively low terminal modulus, especially where the dimensionally stable yarn is further processed into a treated cord.
To overcome at least some of the disadvantages associated with the production of dimensionally stable yarns, polymeric filaments may be spun under high stress conditions to form an undrawn yarn that has crystallinity of 3 to 15 percent and a melting point elevation of 2 to 10 degrees Centigrade as described in U.S. Pat. No. 5,403,659 or U.S. Pat. No. 6,403,006 to Nelson et al. Alternatively, modified process conditions (e.g., using quench delay plus rapid cool to form a yarn with crystallinity of 3 to 13% and a melting point elevation of 2 to 10 degrees Centigrade) may be employed in a process of making dimensionally stable polyester yarn for high tenacity treated cords as described in U.S. Pat. No. 5,630,976 to Nelson et al.
Similarly, Rim et al describe in U.S. Pat. No. 5,397,527 a process for production of dimensionally stable yarns using quench delay plus rapid cool to form a partially oriented yarn of birefringence of at least 0.030, which is then hot-drawn to a drawn dimensionally stable yarn. However, while such processes typically improve various properties of undrawn yarns, drawing such yarns frequently remains problematic, especially where crystallinity of such yarns is relatively high.
Thus, although there are numerous processes for production of dimensionally stable undrawn yarns known in the art, all or almost all of them suffer from various disadvantages. Therefore, there is still a need to provide improved yarns and processes, especially where such yarns are dimensionally stable.